By adding the option --readonly we can restrict these for read only access. Use the --transient option if you only want the shares to appear in the present session but not persistent for follwing sessions. There are some limitations for shared folders (see this question for details). If prerequisites are met we may mount these shared folders manually by running the following commands in the guest:

Of course we can also use different mount options to mount as read/only or mount with read access only to root.

Auto-Mount through Virtual Box Manager

In case we enabled auto-mounting on creating a shared folder from the Virtual Box Manager those shared folders will automatically be mounted in the guest with mount point /media/sf_<name_of_folder>. To have access to these folders users in the guest need to be a member of the group vboxsf.

Actually there is an easy way to do that. First you have to install the extension pack for VirtualBox. After that and restart your virtual machine, install in your guest Ubuntu GuestAdditions (you can do it mounting the iso which is on /media or LeftControl+D).

After that, reboot and try to access to /media/sf_your_shared_folder_name. If you still don't have access, that's mean you don't belong to the vboxsf group, as Nilo points earlier.